Ceramic high-temperature coloring process

ABSTRACT

Composition for coloring ceramic articles obtained from ceramic mass containing TiO 2  as such, in shades ranging from yellow to orange, consisting of an aqueous or hydroalcoholic solution to be applied to the surface to colored before final firing and containing soluble compounds of chromium/antimony or of chromium/zinc or chromium/zirconium or chromium/manganese mixtures.

FIELD OF THE INVENTION

The present invention relates to compositions for colouring ceramicproducts and the relevant colouring process.

In particular the compositions according to the present inventionconsist of aqueous or hydroalcoholic solutions of an inorganic salt ororganic derivative of chromium in combination with an inorganic salt ororganic derivative of a metal selected from antimony, zinc, zirconium,manganese, which allow the obtainment of ceramics in a particularcolour, by using ceramic supports which contain some amount of TiO₂added as such to the ceramic mass before the molding. Furthermore, saidsolutions may also be used in combination with solutions of othermetallic cations to obtain particular shades.

STATE OF THE ART

The use of coloured ceramics as well as the compositions and processesmeant for obtaining said colours have been known since long. One of themost common methods is based on the addition of powdered pigments, inparticular inorganic oxides and mineral colouring matters, to theceramic mass (vitrified stoneware) before firing. The ceramic productobtained is coloured through its whole thickness, with considerableconsumption of colouring matter, which is the most expensive component.

Considering that colouring inside ceramic articles is superfluous, thematerial being not transparent, studies were conducted to developmethods for colouring the ceramics surface only, with a considerablesaving of costly materials.

A method developed to this end is based on pigments addition to thesurface of ceramics (in particular tiles), already in the mold, beforefinal pressing and firing.

Said technique is especially used for vitrified stoneware tiles as itproduces a 2 to 3 mm thick surface layer that may also be smoothed. Saidmethod is material-saving, the colour being imparted only to a thinlayer of the ceramic product; however, it is hardly viable as itrequires devices for metering and feeding, in the molds for ceramics,the amounts and types of pigments needed for obtaining the desiredproducts. Moreover, it does not allow the obtainment of particularlycomplex products.

Another process consists in causing the surface of the ceramic materialto absorb an aqueous solution of inorganic salts or of metal organicderivatives (as described, e.g., in Sprechsal, vol. 119, No. 10, 1986and in EPA 0 704 411) after partial firing (as disclosed, e.g., inGerman patent No. 2,012,304) or simply after molding and before firing(as disclosed, e.g., in Swiss patent No. 575,894), said inorganic saltsand metal organic derivatives being transformed into colourants, stableat a high temperature, during ceramic firing. The aqueous solution isapplied to the ceramic material e.g. by dipping or spraying or disktreatment or silk-screen process.

The aqueous solution is applied to the ceramic material before finalfiring.

A colouring of this type is particularly advantageous as it allows theobtainment of very thin coloured layers. It is widely used in the caseof flat articles (such as for example tiles for floors and walls).

A further problem set by the use of colours in aqueous solution is thedepth to which the colouring matter penetrates into the ceramicmaterial. In fact, experimental evidences have been provided that thepenetration depth depends on various parameters, such as the colouringsolution viscosity and surface tension, the application temperature, thequantity of water sprayed on the manufacture after application of thecolouring solution, the application technique.

This last parameter, i.e. the application technique, is of greatimportance: in fact, whereas the amount of colouring solution that 10may be applied by disk treatment or spraying may be as high as 400 to600 g/m2, by silk-screen techniques it may generally be 100 to 200 g/m2.

Silk-screen techniques are particularly asked for being the only onesallowing the obtainment of graphics, drawings and decorations, andrequiring lower amounts of colourant. Colourant penetration into theceramic material before firing takes place with relatively high amountsof water or of other products after application of the colouringsolution. However, the resulting colours are less intense thanobtainable by the other techniques.

Colouring matter penetration into the ceramic material is of particularimportance in the case of smoothed vitrified stoneware tiles. The phrase"smoothed vitrified stoneware" is used herein to mean a vitrifiedstoneware whose surface is abraded by diamond wheels through a thicknessof 0.8 to 1.5 mm, smoothed and felt-polished to a glassy surface.Obviously, colouring of stoneware to be polished after firing willrequire colourant penetration to a depth of 1.6 mm min.

Technical problem

Since colouring of ceramic materials by disk treatment or spraying iseasy to carry out, the ceramic industry is obviously highly interestedin finding new substances compatible with said techniques.

The technical problem to be solved is that said new substances be easilytransformed into colorants stable at a high temperature, be capable ofimparting the desired colour in the desired shade to the manufacture,without too great a consumption of colourant, and that colorantpenetration into the ceramic material be deep.

Unfortunately, very few are the colours available so far. In particular,the lack of yellow and orange shades is especially felt by the industryof vitrified stoneware, new aesthetic solutions being always needed.

Moreover, since yellow is a primary colour, the lack of productsimparting the aforesaid shades makes it impossible to obtain many othercolours.

DETAILED DESCRIPTION OF THE INVENTION

The Applicant, who has many years' experience in the production and saleof colouring matters for ceramic tiles, has now found that aqueous orhydroalcoholic solutions of inorganic salts or organic derivatives ofchromium in combination with inorganic salts or organic derivatives of ametal chosen from antimony, or zinc, or zirconium or manganese, give,after firing, shades ranging from yellow to orange yellow, to orange, toorange beige, to Havana orange.

The resulting colours depend on the weight ratio antimony/chromium,zinc/chromium, zirconium/chromium, manganese/chromium in the solution aswell as on the TiO₂ content, and the Al(OH)₃ content, if any, in theceramic support (before firing).

Said aqueous or hydroalcoholic solutions are particularly suitable forcolouring vitrified stoneware tiles, either smoothed or not, also bysilk-screen application techniques.

It is an object of the present invention to provide aqueous orhydroalcoholic solutions of inorganic salts or organic derivatives ofantimony and chromium, or zinc and chromium, or zirconium and chromium,or manganese and chromium, for treating molded articles made of ceramicmass added before molding, with 0.5% and up to 10% (preferably at least1%) by w. (on the dry material) of TiO₂ and optionally also with 1% to8% by wt. of Al(OH)₃, (dry basis) or of equivalent amount of Al₂ O₃, toobtain, after firing, a coloring of ceramic articles in yellow, ororange yellow or orange or beige orange or orange Havana.

It is a further object of the present invention to combine theaformentioned inorganic salts or organic derivatives of chromium andantimony, or chromium and zinc, or chromium and zirconium, or chromiumand manganese, with other inorganic salts or organic derivatives ofmetals already known for colouring ceramic supports such as V, Ni, Co,Fe to obtain not yet available colours or shades.

Although the salts of sulphuric, hydrochloric, and nitric acids areparticularly economic and suitable for obtaining the desired colours,they bring about the formation of corrosive gases during firing;consequently, kilns provided with flue gas abatement devices arerequired. It is, therefore, preferred to use, whenever possible, organicderivatives that, during firing, are thermally decomposed to give waterand carbon dioxide. Particularly suitable are the salts of mono- orpolycarboxylic organic acids containing 1 to 18 carbon atoms, with 1 to5 substituents, if any, in the aliphatic chain, which may be hydroxy,amino, mercapto groups.

The following carboxylic acids are reported by way of example and not oflimitation: acetic, formic, propionic, butyric, lactic, glycolic,tartaric, citric, oxalic, maleic, citraconic, fumaric, gluconic,aminoacetic, aminoadipic, aminobutyric, aminocaproic, aminocaprylic,2-amino-4-hydroxybutyric, aminoisobutyric, aminolevulinic, thioglycolicacids.

According to the present invention, suitable organic derivatives arealso those with polymeric polycarboxylic acids, such as polymers andcopolymers of acrylic or methacrylic acid and vinyl ether copolymerswith maleic anhydride and acrolein.

The amounts of inorganic salts and organic derivatives to be added tothe ceramic material prior to firing to obtain the desired colour mustbe such as to secure the application on the surface of the material tobe coloured, of the following colouring ingredients:

in the case of the chromium/antimony mixtures an amount of Sb between 1and 90 g/m² (preferably 3÷48 g/m²) and of Cr between 0.1 to 30 g/m²(preferably 0.8÷15 g/m²)

in the case of the chromium/zinc mixtures an amount of Zn between 1 and60 g/m² (preferably 2÷30 g/m²) and of Cr between 0.05 and 20 g/m²(preferably 0.05÷12 g/m²)

in the case of the chromium/zirconium mixtures an amount of Zr between 1and 60 g/m² (preferably 2÷30 g/m²) and of Cr between 0.05 and 20 g/m²(preferably 0.05÷12 g/m²)

in case of chromium/manganese mixtures an amount of Mn between 1 and 40g/m² (preferably 2÷32 g/m²) and of Cr between 0.05 and 20 g/m²(preferably 0.05÷12 g/m²)

all the amounts above reported being expressed as element. At a higherchromium concentration, the colour gradually turns from orange yellow toorange beige. For example, at an antimony/chromium ratio of 6, thecolour is yellow with a light shade of orange, at a ratio of 3.3, thecolour is orange yellow, and at a ratio of 1.5 the colour is intenseorange.

The typical process for the application of the colouring compositionsaccording to the invention consists of the following essential steps:

a) drying at 100° C. of the molded article to be coloured to a waterresidue of 0.5% by wt. max.;

b) treatment of the article coming from preceding step with thecolouring composition in aqueous solution in an amount of 30 to 600 g/m2of the final coloured surface;

c) equalization of the article coming from preceding step at roomtemperature for a period of 8 hours to homogenize the solutionabsorption;

d) subsequent firing according to the normal ceramic cycle.

The following intermediate operative steps are optional.

between step a) and step b) it is carried out a pre-treating of thedried article from step a) with water in an amount of max. 300 g/m² ofthe article surface

between step b) and step c) it is carried out a post-treatment of thearticle coming from step b) with water in amount of max. 300 g/m² of thearticle surface.

The aqueous solution of the colouring agent according to the inventioncan be converted in a paste suitable for the application on the ceramicsurface through the silk-screen method, by addition of a thickeningagent for instance glucomannan in amount of the order of 1% by w.

The ceramic mass compositions (% by wt.) used for the tests are reportedbelow:

A) SiO₂ =64.4%; Al₂ O₃ =21.8%; K₂ O=3.8%; Na₂ O=0.8%; CaO=0.6%;MgO=0.1%; TiO₂ =0.4%; Fe₂ O₃ =0.2%; ZrSiO₄ =5%; H₂ O balance to 100%.

In the analytical data of the ceramic mass type A is reported thecontent of TiO₂ =0.4%. In fact this datum is an analytical expressionwhich merely means that the titanium present in different chemicalforms, for instance as Ti silicate, corresponds to an amount of TiO₂ of0.4%.

B) SiO₂ =64.4%; Al₂ O₃ =21.8%; K₂ O=3.8%; Na₂ OO=0.8%; CaO=0.6%;MgO=0.1%; TiO₂ =5.4%; Fe₂ O₃ =0.2%; H₂ O balance to 100%.

The ceramic mass B has been obtained by introducing in a ceramic masstype A 5% by w. of TiO₂ instead of 5% of zirconium silicate.

C) SiO₂ =64.4%; Al₂ O₃ =21.8%; K₂ O=3.8%; Na₂ O=0.8%; CaO=0.6%;MgO=0.1%; TiO₂ =3.4%; Fe₂ O₃ =0.2%; H₂ O balance to 100%.

The ceramic mass C has been obtained by introducing in the ceramic mass,3% by w. of TiO₂ instead of 5% of zirconium silicate.

EXAMPLES 1 TO 30

A series of colouring tests were conducted according to the followingprocess.

Some 33 cm×33 cm tiles were press molded, dried at 100C to a waterresidue of 0.1% (weight loss after 4 hrs at 120° C.), cooled to roomtemperature, sprayed with 50 g/m2 distilled water (pretreatment) andwith 250 g/m2 of colouring matter in aqueous solution (treatment).

Once the tiles had undergone said treatment, they were allowed to standat room temperature for 8 hrs (equalization) and fired in a roller kilnaccording to a standard cycle for vitrified stoneware (at a temperatureof 1200° C. max.).

After firing, one tile was cut and colour penetration was measured by anoptical microscope. Another tile was smoothed by diamond wheel abrasionthrough 1.2 mm and the colour layer thickness was recorded. Theparameters used in the various tests and the results obtained arereported in Table 1.

                  TABLE 1                                                         ______________________________________                                                    (3)                                                                           Content in                                                                    % by w. of                                                                    cations                                                                       (as element)                                                                  in the                                                            (1) (2)     solution  (4)   (5)      (6)                                      ______________________________________                                         1  B            Sb  2.8  1.8   orange yellow                                                                          orange yellow                                         Cr  0.85                                                      2  C            Sb  2.8  1.8   orange yellow                                                                          orange yellow                                         Cr  0.85                                                      3  A            Sb  2.8  1.8   colourless                                                                             colourless                                            Cr  0.85                                                      4  B            Sb  2.5  1.8   orange   orange                                                Cr  1.7                                                       5  C            Sb  2.5  1.8   orange   orange                                                Cr  1.7                                                       6  A            Sb  2.5  1.8   light beige                                                                            light beige                                           Cr  1.7                                                       7  B            Sb  2.2  1.8   deep orange                                                                            deep orange                                           Cr  2.55                                                      8  C            Sb  2.2  1.8   deep orange                                                                            deep orange                                           Cr  2.55                                                      9  A            Sb  2.2  1.8   sand     sand                                                  Cr  2.55                                                     10  B            Sb  1.9  1.8   beige orange                                                                           beige orange                                          Cr  3.4                                                      11  C            Sb  1.9  1.8   beige orange                                                                           beige orange                                          Cr  3.4                                                      12  A            Sb  1.9  1.8   sand     sand                                                  Cr  3.4                                                      13  B            Zn  5.4  1.8   beige yellow                                                                           beige yellow                                          Cr  0.85                                                     14  C            Zn  5.4  1.8   beige yellow                                                                           beige yellow                                          Cr  0.85                                                     15  A            Zn  5.4  1.8   very light beige                                                                       colourless                                            Cr  0.85                                                     16  B            Zn  4.8  1.8   Havana yellow                                                                          Havana yellow                                         Cr  1.7                                                      17  C            Zn  4.8  1.8   Havana yellow                                                                          Havana yellow                                         Cr  1.7                                                      18  A            Zn  4.8  1.8   Havana   light Havana                                          Cr  1.7                                                      19  B            Zr  5.4  1.8   beige yellow                                                                           beige yellow                                          Cr  0.85                                                     20  C            Zr  5.4  1.8   beige yellow                                                                           beige yellow                                          Cr  0.85                                                     21  A            Zr  5.4  1.8   very light beige                                                                       colourless                                            Cr  0.85                                                     22  B            Zr  4.8  1.8   Havana yellow                                                                          Havana yellow                                         Cr  1.7                                                      23  C            Zr  4.8  1.8   Havana yellow                                                                          Havana yellow                                         Cr  1.7                                                      24  A            Zr  4.8  1.8   Havana   light Havana                                          Cr  1.7                                                      25  B            Mn  5.4  1.8   Havana yellow                                                                          Havana yellow                                         Cr  0.85                                                     26  C            Mn  5.4  1.8   Havana yellow                                                                          Havana yellow                                         Cr  0.85                                                     27  A            Mn  5.4  1.8   light beige                                                                            light beige                                           Cr  0.85                                                     28  B            Mn  4.8  1.8   orange Havana                                                                          orange Havana                                         Cr  1.7                                                      29  C            Mn  4.8  1.8   orange Havana                                                                          orange Havana                                         Cr  1.7                                                      30  A            Mn  4.8  1.8   light Havana                                                                           light Havana                                          Cr  1.7                                                      ______________________________________                                         (1) example number                                                            (2) ceramic mass used                                                         (3) Content % by w. of cations (i.e. metal compounds) expressed as            elements in the used solution. Antimony is used in form of Sb                 hydroxysuccinate stabilized with potassium (weight ratio Sb/K = 3)            Chromium is used in form of Cr trisetanoate                                   Zirconium is used in form of Zr bishydroxy bisetanoate                        Manganese is used in form of Mn bisetanoate                                   Zinc is used in form of Zn bisetanoate                                        (4) colour penetration (mm)                                                   (5) surface colour before the smoothing                                       (6) surface colour after the smoothing                                        Note                                                                          The tests with the ceramic mass type A wherein TiO.sub.2 as such has not      been added as reported above, are given for comparison purpose: from thes     tests it results a colouring in some cases absolutely insufficient, in        other cases too faint and in any case not corresponding to the specific       colouring according to the invention.                                    

What is claimed is:
 1. Process for obtaining ceramic articles coloured,on the surface and below the surface for at least 1.6 mm, in coloursranging, after firing, from yellow to orange, said process comprisingadding to a ceramic mass before moulding from 0.5% to 10% by weight ofTiO₂ based on the dry material and treating the moulded articles in aprocess according to the following operative steps:a) drying at 100° C.the molded article to be coloured to a maximum water residue of 0.5% byweight; b) treating the article coming from the preceding step with anaqueous solution of an organic or inorganic Cr compound and an organicor inorganic compound of an element selected from Sb or Zn or Zr or Mnor their mixtures, said solution containing compounds of the aboveelements in such concentrations that when applied to the ceramic surfacein an amount of 30 to 600 g/m² secures the application of metal elementsbased on the total weight of elements present as follows: a) 0.1 to 30g/m² of Cr and 1 to 90 g/m² of Sb, or b) 0.05 to 20 g/m² of Cr and 1 to60 g/m² of Zn, or c) 0.05 to 20 g/m² of Cr and 1 to 60 g/m² of Zr, or d)0.05 to 20 g/m² of Cr and 1 to 40 g/m² of Mn c) equalization of thearticle coming from the preceding step, at room temperature for a periodof 8 hours; d) subsequent firing according to a ceramic cycle. 2.Process according to claim 1 wherein Al(OH)₃ is added to the ceramicmass in an amount of 1%-8% by wt based on the dry material.
 3. Processaccording to claim 1 wherein the compound of Cr, Sb, Zn, Zr and Mn aresalts of mono- or polycarboxylic organic acids containing 1 to 18 carbonatoms, optionally having 1 to 5 substituents in the aliphatic chainselected from the group consisting of hydroxy, amino and mercaptogroups.
 4. Process according to claim 1 wherein between step a) and stepb) the following intermediate step is carried out:a.1) pre-treating thedried article from step a) with water in an amount up to a maximum of300 g/m² of said article surface.
 5. Process according to claim 1,wherein between step b) and step c) the following intermediate step iscarried out:b.1) post-treatment of the article from step b) with waterin an amount up to a maximum of 300 g/m² of said article surface. 6.Process according to claim 1, wherein both pre-treatment step and apost-treatment step are employed comprising pre-treating the driedarticle obtained from step a) with water in an amount up to a maximum of300 g/m² of said article surface and post-treating the article from stepb) with water in an amount up to a maximum of 300 g/m² of said articlesurface.
 7. Process for colouring ceramic articles according to claim 1wherein after the firing the ceramic articles, the articles aresubjected to a partial abrasion up to a thickness of 0.8 to 1.5 mm andto a final smoothing of the surface.